Astronomy

What elements and compounds would be present in a red gas giant?

What elements and compounds would be present in a red gas giant?


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What would give a gas giant a red color (dark or light)?


It's a more complicated question than it appears. Also, a couple points are in order.

First, you're thinking about gas giant color is wrong. It's not the ingredients as much as the temperature. The ingredients don't change all that much. They change some, but gas giant color has more to do with temperature.

Two, color is a very narrow perspective. We see, only a narrow band of the spectrum.

NASA brings a variety of cameras on their missions, Infra Red, Radio, UV as well as visible light. See here.

Also, many photos that we see of planets are doctored to make the variations stand out more visibly, and I think it's great that NASA does this.

And three, the planet's star can affect it's color. Color is different under a blue star compared to a red dwarf, so there's 2 colors to consider, color under a white light and color under the planet's star.

All that said, color's still interesting and fun and it tells us something. Using visible light only, many gases are largely transparent, but that doesn't mean they're colorless. Any gas will reflect and diffract some sunlight and gas giants have atmospheres thousands of miles thick. There's no such thing as a transparent gas giant planet, with an earth like atmosphere that you can see through.

Hydrogen and Helium are both highly transparent to visible light but bluish when thick enough. (Neptune gets it's blue color from a combination of it's hydrogen and helium and trace amounts of Methane ice).

Upper-atmosphere temperature is also very important. Saturn, Uranus and Neptune are cold enough that mostly their color is defined by ice, which tends to be more reflective, giving them lighter and generally considered "prettier" colors. Uranus and Neptune's methane ice give them their blue/green and more pure blue colors. Saturn is a little too warm for Methane ice, but it has ammonia ice giving it a lighter color and other gases in it's upper atmosphere giving it some darker yellow and orange colors.

Jupiter's bands can be color-differentiated by where there is abundant ice high in it's atmosphere and where there is more gas. The darker bands are warmer, generally circulating upwards from deeper in the planet, like Hadley cells on Earth, driven in part by Jupiter's fast rotation and strong Coriolis effect and significant internal heat. It's lighter bands are colder, where ice reflection is the primary factor in it's color. Jupiter also has some complex hydrocarbons in it's upper atmosphere due to chemistry, intense lightning and atmospheric mixing.

So there's quite a few difficulties in answering this because a gas giant planet's color depends on many factors, How well it's upper atmosphere circulates, it's temperature and what ices, if any, can form in it's upper atmosphere. Any trace elements that might carry color, even a 1 part in tens of thousands, can be enough for a dominant color if the other gases are mostly transparent. Take Earth for example - clouds are a very small part of the atmosphere by mass but they change the color of the sky (viewed from space) significantly. This makes predicting a gas giant planet's color very tricky and borderline guess-work, because one trace gas can be enough to change it.

Gas giants are mostly hydrogen and helium in their upper atmospheres, but, as noted above, it's often the other gases, ices or trace gases, not the hydrogen and helium that will give them their color.

I always thought Jupiter's "great red spot" was misnamed. It looks more like an orange-brown to me. But if that counts as red, then red might be relatively common.

It might be UV rays and a trace gas, ammonium hydro-sulfide, that gives Jupiter's spot it's color. See article here. I gather there's still some debate on that question.

Here's a question with a nice answer on the colors of the planets. Venus is unusually bright of the reflective SO2 high in it's atmosphere.


Acksblog

Fig. 1. The band of Jupiter’s atmosphere wrapped around its equator, increases its apparent equatorial diameter.

Oblateness
In addition to their low densities, planetary scientists believe Jupiter and Saturn are ‘gas giants’ because of their oblatness. However, cyclic catastrophism explains their high oblatnesses in the context of their highly deuterated Methane Gas Hydrate composition (MGH). Oblatness O = (equatorial diameter (DE) – polar diameter (DP))/ (equatorial diameter (DE)). The oblatenesses of the outer planets are:

Jupiter = 0.06487, Saturn = 0.09796, Uranus = 0.02293, Neptune = 0.01708, Pluto = 0.0000

The difference between the high oblateness of Jupiter and Saturn versus Uranus, Neptune and Pluto has resulted in planetary scientists deciding that the former are ‘gas giants’ and the other three ‘ice giants’. However, they are all ice giants – methane gas hydrate (MGH) is a form of ice which forms in the presence of ample methane. The impact on Jupiter 6,000 years BP is what made both Jupiter and Saturn appear to be gaseous.

Fig. 2 IR image of Jupiter
with Great Red Spot on the left and the fusion source on the right.

A fusion reaction, in which protons and deuterons fuse to produce bare nuclei of a light isotope of helium, called a helion, with a kinetic energy of 5 MeV (expressed as p + d → 3 He ++ + 4.98 MeV), marked by the Great Red Spot 50,000 km to the west, is continuously forming the visible atmosphere of Jupiter. The local heat of the fusion reaction is releasing the full range of the known elements enclosed in the clathrate MGH. Planetary scientists have only identified a few, such as nitrogen, oxygen and sulfur, which they believe are from comets that impacted Jupiter. Why are the normal elements not identified? Because they form high-temperature compounds in the hot vortex of 10 32 helions/second rising to the Great Red Spot, which we do not usually find in Earth-like conditions. As the vortex rises and cools these compounds condense and crystallize forming solid particulate aerosols, which are carried upward within the vortex formed by the helions and sprayed out into the upper atmosphere, consistent with the Galileo atmospheric probe findings that the atmosphere above the cloud-tops is denser and warmer than ‘expected’. One such compound is CS (carbon sulfide) which forms tiny red crystals, causes the red tints and colors the GRS. These aerosols form the tinted clouds we see, Jupiter’s Jujitsu Belt, which cannot be identified by spectroscopy. ‘Gas giant’ scientists claim that the clouds ‘may be’ forms of ammonia snow.

This visible atmosphere is continuously being manufactured by the ongoing fusion reaction and the released aerosols are continuously settling to the surface. JunoCam has revealed that these clouds only extend as far north and south to

70 degrees latitude, probably because they are kept aloft partially by the centrifugal force of Jupiter’s rapid rotation. Poleward of these limits, JunoCam reveals the true solid surface of Jupiter, with cyclones revolving about low pressure centers, as on Earth, accentuated by the much greater Coriolis effect on Jupiter. This rotation is opposite from that of the Great Red Spot, which is not a storm. Since the fusion reaction was stronger in the past, a strip of desert-like terrain covered with atmospheric particles can be seen just poleward of the Jiu Jitsu belt. Also, because the fusion reaction, the origin of the tinted aerosol atmosphere, the Great Red Spot, is at 22 degrees South Latitude, the atmosphere extends further south than north (Figure 3.), and the south pole area is probably warmer than the north.

Fig. 3. ‘Comet’ 67P, is actually a ‘Juno asteroid’ many millions of which are present throughout the solar system, the largest is Pluto.

A slowly declining fusion reaction in the crater of the enormous impact explosion on Jupiter 6,000 years ago has ejected an uncountable number of bodies into all parts of the solar system. Many millions of these comprise the main asteroid belt, the Kuiper belt, the Jupiter trojans and the Kreutz sungrazers. These were formed from a hot fusion plume on Jupiter in a weightless environment and condense into low density bodies, one example of which is ‘comet’ 67 P Churyumov Gerasiamento. They are are not comets, but asteroids, comprising the complete abundance of solar system elements which, because they were all outward from the plume with similar velocities and directions, ‘splatted’ and adhered forming larger bodies, as observed in 67P.

Fig. 4. NASA Cassini probe imaged ‘spokes’ showing new material still being blasted from Saturn into the rings.

Since Saturn is the closest massive planet to Jupiter, it has been bombarded with thousands of these asteroids in the last 6,000 years. The solid body of Saturn also comprises highly deuterated Methane Gas Hydrate so these impacts on its surface produce secondary fusion explosions similar to those produced by the impacts of the larger Shoemaker-Levy 9 fragments on Jupiter, although modern science does not yet realize that fusion explosions were involved (REF). These blast material into the air, primarily water, increasing the thickness of the atmosphere, and into space, adding to the rings to this day. Scientists, judging Saturn’s diameter by the cloud-tops, calculate its oblateness using the enormously expanded equatorial diameter, and calculate a very low density, (δ=0.7), much less than MGH (δ=0.9), resulting in its classification as a gaseous planet.

Images of Saturn captured by the Cassini probe (Figure 4), actually show shadows of new material recently blasted into space by the impacts of Jupiter asteroids on its surface. Most of the mass blasted from the surface is slowed by the thick atmosphere and inflates it even further, giving it the highest oblateness of all the planets in the solar system.

Obliquities
Many impacts of large asteroids from Jupiter also are the cause of the large obliquities (degrees) of the giant planets, the angle of their spin axes relative to the perpendicular to the ecliptic plane, while that of Jupiter is minimal.
Jupiter 3.1, Saturn 26.7, Uranus 97.8, Neptune 28.3, Pluto 122.5.


Содержание

Gas giants may have a rocky or metallic core—in fact, such a core is thought to be required for a gas giant to form—but the majority of its mass is in the form of the gaseous hydrogen and helium, with traces of water, methane, ammonia, and other hydrogen compounds.

Planetary Characteristics [ править ]

Gas giants do not have a well-defined surface their atmospheres gradually become denser toward the core, perhaps with liquid or liquid-like states in-between.

  • These deep interior regions may be composed of metallic hydrogen or metallic helium, kinds of degenerate matter that are still fluidic but behave like electrical conductors.

As a matter of practicality, the diameter of a gas giant is generally measured at the mean altitude where the atmospheric pressure equals 1000 millibars. This corresponds to the mean surface pressure of Terra's atmosphere, which is used as a benchmark. This procedure is very arbitrary: a gas giant's atmospheric pressure varies greatly from region to region, and even the smallest gas giant will have thousands of kilometers of complex, stratified "upper" atmosphere above the measuring point.

  • Thus, terms such as diameter, surface area, volume, surface temperature and surface density effectively refer only to the outermost layer visible from space.
  • The third digit of the "PBG" element of the Universal World Profile indicates how many gas giants are present within the star system.

Gas Giant Life [ править ]

It is possible for lifeforms (and even NILs) to originate on gas giants

Probable Planetary Orbit & Climate [ править ]

Due to the mechanics of system formation, related to the distribution of material within the protoplanetary disk, gas giant worlds most commonly form within the habitable zone and the outer regions of a star system. They are usually found in those regions.

  • Occasionally, celestial mechanics and gravitational forces may cause a gas giant world to migrate inward through the star system. Some, rather than rapidly burning up in the star's photosphere or being flung out of the system, instead settle into a stable orbital position close to the star. These worlds become Hot Jupiters.

The color of a gas giant depends largely on its chemical components and the various compounds and substances that form within it these stain the atmosphere. Colors typically include white, gray, and shades of blue, green, yellow, orange, brown, tan and red. Gas giants look very different if viewed in different spectra such as IR. Gas giants are most commonly monochrome, mottled or banded. A Hot Jupiter may be incandescent.

The upper layers of a gas giant's atmosphere appear similar to the skies of a terrestrial world: the gas mixture is generally transparent, with vast banks of mountainous clouds divided by abyssal chasms and sporadically lit by the flashes of enormous bolts of lightning. The gas mixture becomes denser and more opaque with depth.

  • Radiation belts, shaped by magnetic fields, surround gas giant worlds. Travelling through radiation belts may cause exposure to dangerous levels of radiation.
  • Huge, spectacular auroras may be seen above the polar regions.
  • Many gas giants produce a great deal of electromagnetic noise across a broad spectrum. This may seriously hamper sensor operations and communications.
  • Wind speeds within the upper atmospheric layers of a gas giant can exceed 600 kph: in some atmospheric layers the local windspeed may exceed the speed of sound. Different atmospheric layers have prevailing winds travelling in different directions: the convergence zones between different atmospheric layers may be extremely turbulent.
  • Different chemicals and compounds may precipitate out of the atmosphere at different depths, temperatures and pressures. They manifest as fogs, rain, sleet, snow, or as heavier solids (typically ices and salts).
  • Individual weather events, such as large storms, can cover millions of km² and last for decades or centuries.

Stardust from red giants

A red giant (AGB star) produces heavy elements such as molybdenum and palladium, which form dust (red squares), while elements like cadmium and some palladium escape as gas. Supernova explosions also produce heavier elements and eject them into space as dust (blue triangles) and gas. In the interstellar medium, the stardust mixes with dust grains formed there. In the disc made of gas and dust, more volatile dust grains close to the hot, young sun are destroyed. Stardust from red giants is more resilient than the other dust and so accumulates in regions closer to the sun. The young Jupiter served as a barrier preventing the mixing of material from the inner and regions. Credit: Mattias Ek/Maria Schönbächler

Some of the Earth's building material was stardust from red giants, researchers from ETH Zurich have established. They have also explained why the Earth contains more of this stardust than the asteroids or the planet Mars, which are farther from the sun.

Around 4.5 billion years ago, an interstellar molecular cloud collapsed. At its center, the sun formed around that, a disc of gas and dust appeared, out of which the Earth and the other planets would form. This thoroughly mixed interstellar material included exotic grains of dust: "Stardust that had formed around other suns," explains Maria Schönbächler, a professor at the Institute of Geochemistry and Petrology at ETH Zurich and member of the NCCR PlanetS. These dust grains only made up a small percentage of the entire dust mass and were distributed unevenly throughout the disc. "The stardust was like salt and pepper," the geochemist says. As the planets formed, each one ended up with its own mix.

Thanks to extremely precise measurement techniques, researchers are now able to detect the stardust that was present at the birth of our solar system. They examine specific chemical elements and measure the abundance of different isotopes—the atomic flavors of a given element, which all share the same number of protons in their nuclei but vary in the number of neutrons.

"The variable proportions of these isotopes act like a fingerprint," Schönbächler says. "Stardust has really extreme, unique fingerprints—and because it was spread unevenly through the protoplanetary disc, each planet and each asteroid got its own fingerprint when it was formed."

Iron meteorite that was analysed at the Institute of Geochemistry and Petrology at ETH Zurich. Credit: Windell Oskay/Flickr/CC BY 2.0

Studying palladium in meteorites

Over the past 10 years, researchers studying rock samples from the Earth and meteorites have been able to demonstrate these so-called isotopic anomalies for more and more elements. Schönbächler and her group have been looking at meteorites that were originally part of asteroid cores that were destroyed a long time ago, with a focus on the element palladium.

Other teams had already investigated neighboring elements in the periodic table, such as molybdenum and ruthenium, so Schönbächler's team could predict what their palladium results would show. But their laboratory measurements did not confirm the predictions. "The meteorites contained far smaller palladium anomalies than expected," says Mattias Ek, postdoc at the University of Bristol who made the isotope measurements during his doctoral research at ETH.

Now, the researchers have come up with a new model to explain these results, as they report in the journal Nature Astronomy. They argue that stardust consisted mainly of material that was produced in red giant stars. These are aging stars that expand because they have exhausted the fuel in their core. Our sun, too, will become a red giant 4 or 5 billion years from now.

In these stars, heavy elements such as molybdenum and palladium were produced by what is known as the slow neutron capture process. "Palladium is slightly more volatile than the other elements measured. As a result, less of it condensed into dust around these stars, and therefore there is less palladium from stardust in the meteorites we studied," Ek says.

The ETH researchers also have a plausible explanation for another stardust puzzle: the higher abundance of material from red giants on Earth compared to Mars or Vesta or other asteroids further out in the solar system. This outer region saw an accumulation of material from supernova explosions.

"When the planets formed, temperatures closer to the sun were very high," Schönbächler explains. This caused unstable grains of dust, for instance, those with an icy crust, to evaporate. The interstellar material contained more of this kind of dust that was destroyed close to the sun, whereas stardust from red giants was less prone to destruction and hence concentrated there. It is conceivable that dust originating in supernova explosions also evaporates more easily, since it is somewhat smaller. "This allows us to explain why the Earth has the largest enrichment of stardust from red giant stars compared to other bodies in the solar system," Schönbächler says.


What is Jupiter Made Of?

Composed predominantly of hydrogen and helium, the massive Jupiter is much like a tiny star. But despite the fact that it is the largest planet in the solar system, the gas giant just doesn't have the mass needed to push it into stellar status.

The surface of Jupiter

When scientists call Jupiter a gas giant, they aren't exaggerating. If you parachuted into Jupiter in hopes of hitting the ground, you would never find firm landing. The atmosphere of Jupiter is 90 percent hydrogen. The remaining 10 percent is almost completely made up of helium, though there are small traces of other gases inside.

These gases pile on top of one another, forming layers that extend downward. Because there is no solid ground, the surface of Jupiter is defined as the point where the atmospheric pressure is equal to that of Earth. At this point, the pull of gravity is almost two and a half times stronger than it is on our planet.

Trying to stand on that surface would be impossible, since it is simply another layer of gases. Spacecraft and astronauts would only sink into the mire. A probe or spacecraft traveling farther toward the center of the planet would continue to find only thick clouds until it reached the core.

Jupiter's core

Details about Jupiter's core remain a challenge to find. Scientists think that the dense central core may be surrounded by a layer of metallic hydrogen, with another layer of molecular hydrogen on top.

Scientists aren't certain of just how solid Jupiter's core might be. While some theorize that the core is a hot molten ball of liquid, other research indicates that it could be a solid rock 14 to 18 times the mass of the Earth. The temperature at the core is estimated to be about 35,000 degrees Celsius (63,000 degrees Fahrenheit).

Discussions about Jupiter's core didn't even begin until the late 1990s, when gravitational measurements revealed that the center of the gas giant was anywhere from 12 to 45 times the mass of Earth. And just because it had a core in the past doesn't mean that it still will today &ndash new evidence suggests that the gas giant's core may be melting.

Not quite a star

Like the sun, Jupiter is composed predominantly of hydrogen and helium. But unlike the sun, it lacks the necessary amount to begin fusion, the process that fuels a star. Jupiter would need to be 75 to 80 times more massive than it is at present to be considered a star. If all of the planets in the solar system had formed as part of the gas giant, it still would not have sufficient mass. Still, by itself, Jupiter is two and a half times larger than all of the other planets in the solar system combined.


Giant Storms on Giant Planets

Superimposed on the regular atmospheric circulation patterns we have just described are many local disturbances—weather systems or storms, to borrow the term we use on Earth. The most prominent of these are large, oval-shaped, high-pressure regions on both Jupiter (Figure 10) and Neptune.

Figure 10: Storms on Jupiter. Two examples of storms on Jupiter illustrate the use of enhanced color and contrast to bring out faint features. (a) The three oval-shaped white storms below and to the left of Jupiter’s Great Red Spot are highly active, and moved closer together over the course of seven months between 1994 and 1995. (b) The clouds of Jupiter are turbulent and ever-changing, as shown in this Hubble Space Telescope image from 2007. (credit a: modification of work by Reta Beebe, Amy Simon (New Mexico State Univ.), and NASA credit b: modification of work by NASA, ESA, and A. Simon-Miller (NASA Goddard Space Flight Center)) Figure 11: Jupiter’s Great Red Spot. This is the largest storm system on Jupiter, as seen during the Voyager spacecraft flyby. Below and to the right of the Red Spot is one of the white ovals, which are similar but smaller high-pressure features. The white oval is roughly the size of planet Earth, to give you a sense of the huge scale of the weather patterns we are seeing. The colors on the Jupiter image have been somewhat exaggerated here so astronomers (and astronomy students) can study their differences more effectively. See Figure 1 in Exploring the Outer Planets to get a better sense of the colors your eye would actually see near Jupiter. (credit: NASA/JPL)

The largest and most famous of Jupiter’s storms is the Great Red Spot, a reddish oval in the southern hemisphere that changes slowly it was 25,000 kilometers long when Voyager arrived in 1979, but it had shrunk to 20,000 kilometers by the end of the Galileo mission in 2000 (Figure 11). The giant storm has persisted in Jupiter’s atmosphere ever since astronomers were first able to observe it after the invention of the telescope, more than 300 years ago. However, it has continued to shrink, raising speculation that we may see its end within a few decades.

In addition to its longevity, the Red Spot differs from terrestrial storms in being a high-pressure region on our planet, such storms are regions of lower pressure. The Red Spot’s counterclockwise rotation has a period of six days. Three similar but smaller disturbances (about as big as Earth) formed on Jupiter in the 1930s. They look like white ovals, and one can be seen clearly below and to the right of the Great Red Spot in Figure 11. In 1998, the Galileo spacecraft watched as two of these ovals collided and merged into one.

We don’t know what causes the Great Red Spot or the white ovals, but we do have an idea how they can last so long once they form. On Earth, the lifetime of a large oceanic hurricane or typhoon is typically a few weeks, or even less when it moves over the continents and encounters friction with the land. Jupiter has no solid surface to slow down an atmospheric disturbance furthermore, the sheer size of the disturbances lends them stability. We can calculate that on a planet with no solid surface, the lifetime of anything as large as the Red Spot should be measured in centuries, while lifetimes for the white ovals should be measured in decades, which is pretty much what we have observed.

Despite Neptune’s smaller size and different cloud composition, Voyager showed that it had an atmospheric feature surprisingly similar to Jupiter’s Great Red Spot. Neptune’s Great Dark Spot was nearly 10,000 kilometers long (Figure 7). On both planets, the giant storms formed at latitude 20° S, had the same shape, and took up about the same fraction of the planet’s diameter. The Great Dark Spot rotated with a period of 17 days, versus about 6 days for the Great Red Spot. When the Hubble Space Telescope examined Neptune in the mid-1990s, however, astronomers could find no trace of the Great Dark Spot on their images.

Although many of the details of the weather on the jovian planets are not yet understood, it is clear that if you are a fan of dramatic weather, these worlds are the place to look. We study the features in these atmospheres not only for what they have to teach us about conditions in the jovian planets, but also because we hope they can help us understand the weather on Earth just a bit better.

Example 1: Storms and Winds

The wind speeds in circular storm systems can be formidable on both Earth and the giant planets. Think about our big terrestrial hurricanes. If you watch their behavior in satellite images shown on weather outlets, you will see that they require about one day to rotate. If a storm has a diameter of 400 km and rotates once in 24 h, what is the wind speed?

[reveal-answer q=�″]Show Answer[/reveal-answer]
[hidden-answer a=�″]Speed equals distance divided by time. The distance in this case is the circumference (2πR or πd), or approximately 1250 km, and the time is 24 h, so the speed at the edge of the storm would be about 52 km/h. Toward the center of the storm, the wind speeds can be much higher.

Check Your Learning

Jupiter’s Great Red Spot rotates in 6 d and has a circumference equivalent to a circle with radius 10,000 km. Calculate the wind speed at the outer edge of the spot.

[reveal-answer q=�″]Show Answer[/reveal-answer]
[hidden-answer a=�″]For the Great Red Spot of Jupiter, the circumference (2πR) is about 63,000 km. Six d equals 144 h, suggesting a speed of about 436 km/h. This is much faster than wind speeds on Earth.[/hidden-answer]

Key Concepts and Summary

The four giant planets have generally similar atmospheres, composed mostly of hydrogen and helium. Their atmospheres contain small quantities of methane and ammonia gas, both of which also condense to form clouds. Deeper (invisible) cloud layers consist of water and possibly ammonium hydrosulfide (Jupiter and Saturn) and hydrogen sulfide (Neptune). In the upper atmospheres, hydrocarbons and other trace compounds are produced by photochemistry. We do not know exactly what causes the colors in the clouds of Jupiter. Atmospheric motions on the giant planets are dominated by east-west circulation. Jupiter displays the most active cloud patterns, with Neptune second. Saturn is generally bland, in spite of its extremely high wind speeds, and Uranus is featureless (perhaps due to its lack of an internal heat source). Large storms (oval-shaped high-pressure systems such as the Great Red Spot on Jupiter and the Great Dark Spot on Neptune) can be found in some of the planet atmospheres.


Ultra-Hot Interstellar Gas

While the temperatures of 10,000 K found in H II regions might seem warm, they are not the hottest phase of the interstellar medium. Some of the interstellar gas is at a temperature of a million degrees, even though there is no visible source of heat nearby. The discovery of this ultra-hot interstellar gas was a big surprise. Before the launch of astronomical observatories into space, which could see radiation in the ultraviolet and X-ray parts of the spectrum, astronomers assumed that most of the region between stars was filled with hydrogen at temperatures no warmer than those found in H II regions. But telescopes launched above Earth’s atmosphere obtained ultraviolet spectra that contained interstellar lines produced by oxygen atoms that have been ionized five times. To strip five electrons from their orbits around an oxygen nucleus requires a lot of energy. Subsequent observations with orbiting X-ray telescopes revealed that the Galaxy is filled with numerous bubbles of X-ray-emitting gas. To emit X-rays, and to contain oxygen atoms that have been ionized five times, gas must be heated to temperatures of a million degrees or more.

Figure 5. Vela Supernova Remnant: About 11,000 years ago, a dying star in the constellation of Vela exploded, becoming as bright as the full moon in Earth’s skies. You can see the faint rounded filaments from that explosion in the center of this colorful image. The edges of the remnant are colliding with the interstellar medium, heating the gas they plow through to temperatures of millions of K. Telescopes in space also reveal a glowing sphere of X-ray radiation from the remnant. (credit: Digitized Sky Survey, ESA/ESO/NASA FITS Liberator, Davide De Martin)

Theorists have now shown that the source of energy producing these remarkable temperatures is the explosion of massive stars at the ends of their lives (Figure 5). Such explosions, called supernovae, will be discussed in detail in the chapter on The Death of Stars. For now, we’ll just say that some stars, nearing the ends of their lives, become unstable and literally explode. These explosions launch gas into interstellar space at velocities of tens of thousands of kilometers per second (up to about 30% the speed of light). When this ejected gas collides with interstellar gas, it produces shocks that heat the gas to millions or tens of millions of degrees.

Astronomers estimate that one supernova explodes roughly every 100 years somewhere in the Galaxy. On average, shocks launched by supernovae sweep through any given point in the Galaxy about once every few million years. These shocks keep some interstellar space filled with gas at temperatures of millions of degrees, and they continually disturb the colder gas, keeping it in constant, turbulent motion.


What elements/compounds give Jupiter its colors?

The atmosphere is mostly hydrogren and helium, with some trace elements. What gives color to those swirling bands?

The short answer: Hydrocarbon hazes, Rayleigh scattering, and some unknown compound.

The long answer: For the primary whites and browns that cover most of the planet, you need to realize that almost everything you see when you look at Jupiter is ammonia clouds, which on their own are bright white. Some latitudes are regions of upwelling (zones), and have high ammonia cloud-tops, while other latitudes are regions of downwelling (belts), and have low ammonia cloud-tops. In between these high and low heights sits a thick brown hydrocarbon haze, very chemically similar to smog. The cloud-tops in the zones are sticking up above most of the haze and thus appear fairly white. The cloud-tops in the belts, though, lie below the haze layer, and thus appear colored brown by the overlying haze.

For the occasional bluish regions seen just to the north and south of the equator, these are some of the rare cloud clearings that occur in very strong downwelling regions. We're actually peering through the ammonia top cloud layer, and perhaps even down through the ammonium hydrosulfide middle cloud layer and the bottom water cloud layer. So, in those regions we're looking at just clear air, which has the exact same color as it does one Earth, blue. This is entirely due to Rayleigh scattering, the same reason that Earth's sky is blue.

Then there's the reds, notably in Jupiter's Great Red Spot, although also occasionally seen in another big vortex here and there. As of right now, we don't actually know what makes the Great Red Spot red - this is generally known as the Jovian chromophore problem. Since this color is only seen in very large vortices, it's believed to be caused by some mixture of compounds already present on the planet getting pushed very high in the atmosphere by these vortices. In three dimensions, the Great Red Spot is essentially shaped like a wedding cake, so the cloud-tops at the center of the spot are at very high altitudes where there's a lot more ultraviolet light. You can end up producing all kinds of odd substances through UV photochemistry of trace substances in the atmosphere, and the working hypothesis at this point is that it's some kind of imine.


Meet Pollux, the Red Giant with a Planet

By: Daniel Johnson September 3, 2018 2

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Pollux Vitals

Physical Characteristics

The constellation Gemini is well known by many backyard astronomers and for good reasons. Besides its fame as the radiant of the annual Geminids meteor shower (not to mention the location where Uranus and Pluto were first discovered), Gemini is an easy-to-find constellation that boasts a pair of respectably bright stars: Castor and Pollux. Both are interesting in their own ways, but in this article we’ll explore Pollux.

The Gemini twins overlook Procyon. (The star cluster left of Pollux and above Procyon is M44, the Beehive in Cancer.)
Akira Fujii

Pollux is a red giant star that has exhausted its supply of hydrogen, and is now fusing helium into carbon and other elements. Like other red giants, this process causes Pollux to become cooler than our Sun — hence its orange color — and much larger: 10 times the diameter of the Sun. At magnitude 1.16, Pollux is among the 20 brightest stars in either hemisphere. All of this makes it an interesting object, but another fact that sets Pollux apart is its exoplanet, discovered by its gravitational pull on its host star.

Exoplanets are no longer the rare objects they were decades ago there are now thousands of known (and suspected) exoplanets. But many of them belong to dim or faraway stars. Exoplanets orbiting bright, familiar stars aren’t as common, so the one near Pollux is a delightful exception. Originally known as “Pollux b,” this exoplanet is now called Thestias. A gas giant with at least twice Jupiter’s mass, it orbits Pollux at a distance of 1.65 astronomical units — a little farther from its star than Mars is from the Sun.

While the star is currently twice the Sun’s mass, it’s expected to puff off enough of its outer layers to enable its core to collapse into a white dwarf.

Origin / Mythology

Naturally, the most famous bit of mythology surrounding Pollux is the Greco-Roman story of the twin brothers Pollux and Castor, who sailed with Jason on the voyage for the Golden Fleece. But the idea that the constellation — and the stars Pollux and Castor in particular — depict twins isn’t an exclusively Greek concept the idea was present in Babylonia as well. In India and North America, on the other hand, the two figures of Gemini symbolized a newly married couple rather than twins, while in China, the two stars represented “yin” and “yang.”

How to See Pollux

For easy evening viewing, Pollux is best observed during the winter months, when it sits high in the southern sky above Orion. Gemini is on the ecliptic, so the Moon (as well as the occasional planet) travels through or near the constellation frequently. Anyone familiar with H.A. Rey’s classic constellation drawings will remember that the stars Pollux and Castor represent the heads of each stick-figure twin. Pollux is the one on the left.

Pollux appears to the upper-left of an old barn in this winter-time photo. Note the Twins' sideways appearance relative to Orion.
Daniel Johnson

Pollux is the brightest star in Gemini, but you wouldn’t know if it from its Bayer designation. Originally created by Johann Bayer in 1603, the Bayer system of star naming often attributes the “alpha” designation to the brightest star in a constellation for instance, Alpha Orionis for Betelgeuse in Orion and Alpha Ursae Minoris for Polaris in the Little Dipper. By this standard, Pollux should likewise be known as Alpha Geminorum, but it’s not. For Gemini, Castor is the Alpha star and Pollux is known as Beta Geminorum. Why?

The reason is that the Bayer system isn’t exclusively based on apparent magnitude, and there are many exceptions (Bayer, it seems, didn’t follow strict rules). In the case of Gemini, the name reversal may have to do with the fact that Castor rises earlier than Pollux. In any case, Pollux is the brighter of the two.

So if you’re looking for something a little different to observe during an upcoming clear night, give Pollux a try. While you’re looking, remember that somewhere near that point of light at least one exoplanet is quietly orbiting. It’s a fun idea to keep in mind!


The Chemical Composition of Stars and the Universe

Of what is the universe made? What are the ingredients for the Cosmic Recipe? If we can answer these questions, we may gain some clue to the history of our universe.

People have long known that the stars are far, far away in the nineteeth century, astronomers finally measured the distances to a few nearby stars with reasonable accuracy. The results were so large -- thousand of trillions of miles -- that most people figured we'd never be able to visit them or learn much about them. After all, we can't go to a star, grab a sample, and bring it back to earth all we can do is look at light from the star. In fact, at least one prominent philosopher and scientist went on the record as saying that we'd never be able to figure out their compositions.

Of all objects, the planets are those which appear to us under the least varied aspect. We see how we may determine their forms, their distances, their bulk, and their motions, but we can never known anything of their chemical or mineralogical structure and, much less, that of organized beings living on their surface .

Auguste Comte, The Positive Philosophy, Book II, Chapter 1 (1842)

But just a few decades after this pessimistic statement, astronomers were starting to identify elements in the solar atmosphere. We now have a good idea about the chemical makeup not only of the stars, but of the entire visible universe.

What about the Earth?

It's easy to figure out the chemical composition of the Earth: just dig up some dirt, and analyze it. Well, maybe it's a bit more complicated than that.

  • the atmosphere
    • 78% nitrogen
    • 21% oxygen
    • 1% other stuff (carbon dioxide, water vapor, argon, etc.)
    • water: 2 hydrogen, 1 oxygen
    • 62% oxygen (by number of atoms)
    • 22% silicon
    • 6.5% aluminum
    • bits of iron, calcium, potassium, sodium, etc.

    If we count the total number of atoms in each component, the atmosphere is by far the least important, and the solid crust by far the most important. One could pretty much ignore the air and the water.

    • a central core
      • mainly iron
      • smaller amounts of nickel and cobalt
      • mostly oxygen and silicon
      • some iron, magnesium, etc.

      Overall, since the core and the mantle comprise most of the atoms of the Earth, the chemical composition of our planet is dominated by iron, oxygen, and silicon.

      The chemical composition of the stars

      In the early days of astrophysics, scientists thought that the stars were probably similar to the Earth in chemical composition. When they passed starlight through a prism and examined the resulting spectrum, they found absorption (and occasionally emission) lines of many elements common here on Earth. For example, here's a portion of the spectrum of Arcturus (taken from a paper by Hinkle, Wallace and Livingstone, PASP 107, 1042, 1995):

      Now, different stars have spectra which look very different (click on image to see larger version):

      Does this mean that the chemical composition of stars varies wildly? Initially, scientists thought the answer was "yes."

      • NOT made up of the same mix of elements as the Earth
      • NOT wildly variable in composition
      • almost entirely hydrogen, in almost all stars
      • 90% hydrogen (by number of atoms)
      • 10% helium
      • tiny traces of heavy elements (everything else)

      The chemical composition of interstellar clouds

      Our galaxy contains not only stars, but also clouds of gas and dust. Some glow brightly, lit up by nearby stars:

      Other clouds appear dark, because they absorb and scatter the light which tries to pass through them:

      It is often easier to determine the composition of nebulae than of stars, since we can see into the center of the nebula. The spectra of these objects show that they, too, are almost completely made of hydrogen and helium, with tiny amount of other elements.

      When we look at different galaxies, we find some variation in the amount of heavy elements. The Milky Way, for example, has more iron (relative to hydrogen) than the Large Magellanic Cloud and the Large Magellanic Cloud has more iron (relative to hydrogen) than the Small Magellanic Cloud.

        hydrogen fuses into helium (in all stars)

      Galaxies with lots of heavy elements must have had several generations of stars, some of which have ejected material from their interiors into the interstellar medium and enriched it with helium and heavy elements.

      Astronomers use the letters X, Y and Z to denote the fraction of material (by mass) which made up by hydrogen, helium, and everything else:

      When we analyze the composition of nebulae in different galaxies, we find a slight correlation between the fraction of helium and the fraction of heavy elements:

      • all galaxies started out with only hydrogen and helium (so Z = 0)
      • a first generation of stars created helium and heavy elements in their cores, and ejected some into the interstellar clouds
      • in some galaxies, a second generation of stars has created even more helium and heavy elements
      • in some galaxies, third or fourth generations of stars have spewed even more helium and heavy elements into interstellar space

      If we can find galaxies which have had little star formation since they were formed, we can use them to measure the primordial abundance of helium, relative to hydrogen.

      The primordial abundances

      In our own corner of the Milky Way, this ratio is currently about 10. There has evidently been quite a bit of nuclear processing of hydrogen into helium by previous generations of stars in our galaxy.

      • Is there any particular reason that galaxies should have started out with a mixture of 12.5 hydrogen atoms for every 1 helium atom?
      • Is there any reason why the initial mixture should contain only hydrogen and helium, with (almost) no heavier elements?
      • Whence came the mixture of oxygen, silicon, iron, etc., which make up the Earth and everything on it?
      • Interactive Solar Atlas software by Sergei O. Naumov. This software must be installed on your local computer before you can use it.
      • The classification of stellar spectra with lots of gory details and descriptions of the different classes
      • Sky and Telescope's description of spectral classes
      • How we know the chemical composition of stars, from Padi Boyd and the Ask a High-Energy Astronomer team

      Copyright © Michael Richmond. This work is licensed under a Creative Commons License.